It is known that articles prepared from a carbonate polymer ("PC") having a higher molecular weight (Mw) possess generally better physical properties than if molded from a lower molecular weight carbonate polymer. Unless otherwise indicated, the references to "molecular weight" herein refer to weight average molecular weights ("Mw") determined on the carbonate polymers using gel permeation chromatography with a bisphenol A polycarbonate standard. Otherwise, viscometry or light scattering can also be used to determine weight average molecular weight. It should be noted that various references, including several that are discussed below, refer to "viscosity average" molecular weight, which is not the same as weight average molecular weight but can be correlated or converted to Mw values.
It is also known that the higher the molecular weight of the carbonate polymers, the more difficult they are to process due to their higher viscosity and corresponding lower melt flow rate. See for example Japanese Patent Publication 03-243,655 (1992) where amounts of a lower molecular weight carbonate polymer are incorporated into a high molecular carbonate polymer composition in attempting to improve the flow and processability properties of such carbonate polymers. However, it is known that the separately prepared higher and lower molecular weight components are difficult to homogeneously blend due to their difference in melt viscosities and do not produce the best possible combinations of polymer processability and molded part physical properties.
It is well known that branched carbonate polymers are commercially available and are known to have greater melt strength and be more shear sensitive in the melt phase than linear polycarbonates. Therefore, the branched polycarbonates are known to be better suited for some uses such as blow molding or thermoforming. In Japanese Patent Publication 60-215,051 (1985), it is shown that branched resins can be incorporated in varying amounts into blends with a high melt flow rate (low molecular weight) linear carbonate polymer to improve the melt strength of the low molecular weight resin. It is also known that additives, such as Teflon polytetrafluoroethylenes, can be added to polycarbonates to provide an increase in melt strength but have drawbacks such as loss in transparency and toughness.
In U.S. Pat. No. 4,912,914 it is proposed that crosslinked or branched polycarbonates can be prepared by incorporating a diester diphenolic monomer into the carbonate polymer backbone, then heat activating the crosslinking reaction. However, since the crosslinking reaction causes the polymer backbone to be cut at the point of crosslinking, the polymers that are taught would be expected to have undesirable levels of low molecular weight polymer byproducts and high molecular weight or crosslinked gel byproducts due to the random and uncontrollable crosslinking. These polymers would also be expected to have higher color levels and reduced hydrolytic stability.
In U.S. Pat. No. 3,770,697 and U.S. Pat. No. 3,652,715 carbonate polymers are provided with thermally activated, terminal or pendant unsaturated imido groups. These functionalized, curable polymers are then taught to be employed independently or in mixtures with other monomers or polymers, as molding compounds, films, laminates, and the like. A cast film that was prepared from a blend of an imido-sunctionalized carbonate polymer composition and an amount of standard, non-functionalized polycarbonate and reacted using an additional chemical initiator compound showed an increase in heat resistance. It has been found, however, that the molecular weight of these products is unstable and that the products have an undesirable discoloration. It has also been found, unfortunately, that such unsaturated imido-functionalized carbonate polymers are not readily produced in interfacial carbonate polymer production processes due to the nitrogen-containing imido groups that must be incorporated. In addition, the initiation and reaction process requires relatively long reaction times.
It is therefore a goal of the present invention to provide improved carbonate polymer compositions which are easily prepared and possess good combinations of processability, stability and physical properties. Preferably such compositions are easily prepared from one or more components produced in an interfacial carbonate polymer process. It is also desired to prepare such compositions without having to incorporate an additional free radical initiating compound such as an organic peroxide or employ radiation techniques which can also lead to polymer decomposition. It is also desired to prepare such improved carbonate polymer blend compositions using a high molecular weight branched carbonate polymer component that can be readily mixed homogeneously into the second component while avoiding unacceptable levels of gel that detract from the balance of the desirable physical, theological and optical properties.